Recently, in the field of semiconductor devices, the minimization, lightening and multi-functionating of the device have been required in proportion as their high integration progress. Various resin compositions were widely used for sealing such semiconductor devices, and it has been required to further improve several characteristics of such resin composition.
For sealing semiconductor devices, epoxy resin, silicone resin, phenol resin, allylic resin such as diallyl phthalate, and the like have been used up to now. Especially, epoxy resin compositions containing phenol novolak type resins as the curing agent have been preferably used because of their good moisture resistance, electrical characteristics at high temperatures, and processability.
In case of sealing semiconductor devices with minute surfaces by using such resin compositions as sealing materials, however, it is likely that their thermal strain stress undesirably rises up during or after the transfer molding process due to the significant difference in thermal expansion coefficients between sealing materials and semiconductor devices. Because of such difference in thermal expansion coefficients the devices become strained and consequently their aluminum pattern and bonding wire may be cut, and also some cracks are formed on the surface or at the inner part of the devices. In addition to the foregoing disadvantages, the aluminum pattern becomes even corroded and the semiconductor device becomes malfunctioning at last.
Hence, in order to lower undesirable internal stress of the semiconductor devices as described above, it has been required, for example, to reduce the difference in the thermal expansion coefficients between the semiconductor device and the sealing material, to increase adhesion between the devices and the sealing material, to prevent the corrosion the corrosion of aluminum pattern by reducing ionic impurities, and so on.
Representative methods for lowering the internal thermal strain stress, which have been conventionally used, are the followings:
First, there is a method of lowering the glass transition temperature (Tg) of the resin composition used as sealing material. However, said method is limited because high temperature impact resistance of the resin composition with low Tg becomes reduced to an undesirable level and also bonding wire of the devices may be cut. Hence, it is necessary to maintain the glass transition temperature (Tg) of more than 150.degree. C.
Another method of lowering internal stress involves reducing the difference in the thermal expansion coefficients between the semiconductor device and the resin composition for sealing, by using inorganic fillers having a low thermal expansion coefficient. However, this method also has some disadvantages: the modulus of the resin composition is increased, viscosity thereof becomes high, and consequently processability thereof is deteriorated, even though the internal stress becomes preferably lowered.
Also, available is the method of lowering modulus of the resin composition by adding a plasticizer such as long chain bisepoxy compounds like polypropylene glycol diglycidyl ether, and diglycidyl ether of bisphenol A (DGEBA) having a long side chain. However, when such plasticizers are used until the modulus sufficiently decreases, the mechanical strength and the glass transition temperature (Tg) become lowered to undesirable levels.
On the other hand, in order to protect the corrosion of aluminum pattern, it is necessary that the concentration of ionic chloride maintain lower than 10 ppm and the amount of hydrolyzable chloride be less than 0.1 part by weight.
Hereupon the present inventors have studied to overcome the foregoing problems and resultingly obtained novel epoxy resin compositions which possess lower modulus and thermal expansion coefficient and maintain high glass transition (Tg), short flash length and excellent processability, so as to be effectively used in sealing of semiconductor devices.